Such induction heating devices are, for example, used as induction coils in induction hobs. The wish is always for ever higher power levels, in particular so as to be able to rapidly carry out the boiling of larger quantities of liquid, for example, for cooking noodles.
At present there is a limit of approximately 3.2 kW, as from which the frequency converters necessary for the power supply exceed the value limits established by standards with regards to harmonics and mains supply reactions. The reason for the severe effects of harmonics and in particular the third harmonic is essentially that the permeability of the magnetic components in the frequency converter changes with the amplitude of the inductor current flowing through the induction coil. At high current amplitudes there is a drop in the permeability of ferrites and the like which are used in an induction coil for field guidance purposes, together with that of the saucepan material. This in turn leads to a change to the inductance of the induction coil during a half-wave of the supply voltage and consequently also the resonant frequency of a series resonant circuit, such as is used in the power supply. Thus, ultimately the power consumption from the mains supply is distorted and its curve diverges from the predetermined supply voltage curve.
Solutions for such induction heating devices or methods for the supply thereof are described in the not previously published DE 10 2005 028 829.4 of the same applicant, where in order to avoid supply reactions an operating frequency of the switching means or the entire frequency converter is increased and then decreased again during a half-wave. However, this operating frequency change is complicated from the control standpoint.